Coating compositions are complex mixtures which often include binders, pigments, surfactants, flow-control agents, and organic solvents. Organic solvents serve a variety of purposes related to viscosity reduction, film formation and adhesion. In spraying paints and coatings, organic solvents reduce their viscosity. This viscosity reduction is needed to enable atomization when the material is sprayed and also to facilitate droplet coalescence on the surface, thus giving a coherent, uniform film. Spray atomization requires a very low viscosity to produce the fine droplets needed for high-quality coatings.
Despite the important role of volatile organic compounds ("VOC") play in the coating's formulation, there has been a considerable effort by the coating formulators and applicators to reduce VOC emissions for both economical and environmental reasons.
A great deal of emphasis has been placed on the development of new coating technologies which will reduce the emission of organic solvent vapors. A number of technologies have emerged as having met most but not all, of the performance and application requirements, and at the same time having met the emission requirements and regulations. They are (a) powder, (b) waterborne, dispersion, (c) waterborne, solution, (d) non-aqueous dispersion, and (e) high solids coatings. Each of these technologies has been employed in certain applications, and each has found a niche in a particular industry. In a majority of cases, the coatings from these new technologies are inferior to the old in one or more important application or performance features.
U.S. Pat. No. 4,923,720 discloses methods and apparatus for the production of the high solid coating formulation in which substantial amounts of the liquid solvent component have been removed and replaced with a non-toxic and environmentally compatible supercritical fluid, such as supercritical carbon dioxide. This coating composition is then sprayed onto a substrate at which time the supercritical carbon dioxide vaporizes to assist spray atomization. In order to produce a coating material solution or formulation with the desired application characteristics, the relative proportion of the liquid composition and supercritical carbon dioxide should be maintained at a predetermined ratio or within a predetermined range. However, one requirement of U.S. Pat. No. 4,923,720 is to control the relative proportion of liquid coating composition and supercritical fluid. The liquid coating composition and supercritical fluid are each introduced into the system by a separate pump. However, the volume of the supercritical carbon dioxide is varied depending upon the system pressure and temperature. This can result in deviation of the supercritical carbon dioxide concentration in the coating formulation, resulting in inconsistent spray characteristics.
U.S. Pat. No. 5,215,253 discloses an improved method and apparatus for forming and dispensing a coating material formulation or solution containing a fluid coating composition and a fluid diluent, such as a supercritical carbon dioxide. The control system opens and closes the supply of supercritical carbon dioxide and/or liquid coating composition in accordance with variation of capacitance in the formulation. The devices requires predetermined set point values to control supercritical carbon dioxide concentration in the coating formulation. However, the correlation between the carbon dioxide concentration in the coating formulation and the values obtained by capacitance sensor can vary significantly depending upon system pressure, temperature and coating formulation. Furthermore, with respect to compositions having both liquid and gas components in a multiple phase solution, it has been found that controlling carbon dioxide concentration is difficult. The signal from the capacitance sensing circuit produces a relatively widely fluctuating signal due to the formation of bubbles. Another deficiency of the apparatus is that the device requires the feed coating capacitance information of formulation before carbon dioxide addition to calculate control set point values with respect to carbon dioxide concentration.
Aforementioned U.S. Pat. No. 4,923,720 discloses an apparatus capable of pumping and proportioning a coating formulation and liquid carbon dioxide. In one embodiment, volumetric proportioning of the coating formulation stream and the supercritical carbon dioxide stream is carried out by means of reciprocating pumps which displace a volume of fluid from the pump during each one of its pumping cycles. One reciprocating pump is used to pump the coating formulation which is slaved to another reciprocating pump which is used to pump the liquid carbon dioxide. The piston rods for each pump are attached to opposite ends of a shaft that pivots up and down on a center fulcrum. The volume ratio is varied by sliding one pump along the shaft, which changes the stroke length.
However, liquid carbon dioxide is relatively compressible at ambient temperature, the temperature at which it is typically stored in a pressurized container. Such compressibility may undesirably cause fluctuations and oscillations of the amount of carbon dioxide that is present in the admixed coating formulation that is to be sprayed. This occurs due to the incompatible pumping characteristics of the relatively non-compressible coating formulation and the relatively compressible liquid carbon dioxide. With the coating formulation, pressure is immediately generated in the reciprocating pump as soon as its volume is displaced. Inasmuch as the liquid carbon dioxide is substantially compressible, a larger volume is needed to be displaced in order to generate the same pressure. Because mixing occurs when the flow of the coating formulation and of the liquid carbon dioxide are at the same pressure, the flow rate of carbon dioxide lags behind the flow rate of the coating formulation.
This oscillation is further accentuated if the driving force operating the pump varies during the operating cycle, such as an air motor changing direction during its cycle. Thus, if the driving force declines, the pressure in the coating formulation flow declines even more rapidly, due to its non-compressibility, than the pressure in the liquid carbon dioxide flow.
Accordingly, the pressures generated in both flows may be out of phase during the pumping. U.S. Pat. No. 4,621,927 discloses a mixture control apparatus controlling a flow rate of a second fluid to be mixed with a first fluid so as to prepare a third fluid having a predetermined concentration. A set point variable of the flow rate of the second fluid is calculated in accordance with the flow rate of the third fluid so as to improve controllability of the apparatus. However, the invention in U.S. Pat. No. 4,621,927 cannot control the mixture of compressible fluid(s) and non-compressible fluid(s) because the thermodynamic properties of the fluids are influenced by variables such as pressure, temperature, and concentration.